Traditional push-to-talk (PTT) communication utilized two or more radio transceiver stations, all tuned to the same channel. When not transmitting, the transceivers received any signal carried over the channel and supplied any received audio to the users. A user wishing to speak pushed a button, which caused that user's transceiver to broadcast audio over the common channel to the other transceiver(s) that shared the channel. Modern public cellular communication systems have evolved in recent years to offer a wide range of voice and data communications services. Many such services emulate traditional voice telephone services offered originally by the public switched telephone network, but the cellular implementation enhances the services with high degree of mobility. In recent years, it has become popular for cellular communication service providers to offer a PTT service to their customers.
The service providers and their equipment vendors have developed a number of different implementations for supporting PTT through wireless mobile networks. Discussion here focuses on implementations of PTT that utilize voice-switched calls through mobile switching centers of a cellular network. In support of such a PTT service implementation, the carrier deploys a switch referred to as a PTT service specific switch (PTT SSS). Voice calls between the PTT service's subscribers are routed through the PTT SSS, which controls the advanced PTT features and differentiates the PTT service from ordinary voice telephone service through the cellular network. As discussed below, problems arising in handling of PTT call for roaming prepaid subscribers. It may be helpful to consider PTT routing and network handling of prepaid calls in some detail, to understand the problems.
For subscribers roaming “off-net” (i.e. receiving service from an MSC belonging to one of the carrier roaming partners), PTT call routing may rely on the use of a mobile station resident PTT client program that stores a centrally administered contact list. Typically, a PTT subscriber places a PTT call by selecting a name corresponding to a contact from the list stored in the subscriber's mobile station, and the resident PTT client program causes this station to place the PTT call. The names corresponding to the contact list may be the names of other PTT users, the names of predefined groups of other PTT users, or a mixture of both. The PTT call goes through the mobile communication network to the PTT SSS.
Even when the selected contact is an individual subscriber, rather than a group, the actual number stored in the contact list for that contact is an alias numbers rather than the actual phone number of the contact, since PTT calls must be routed to the PTT SSS rather than to the called contact's Home MSC. While the subscriber identifies contacts by their names and actual corresponding telephone numbers for establishment of the contact in the system, the alias numbers are managed and translated through the PTT SSS. The alias numbers for the contacts on the subscriber's list are downloaded into the PTT client program in the mobile station, and the PTT client uses one of those alias numbers to make each PTT call. The alias numbers are limited in extent, the same alias numbers being assigned to a plurality of subscribers and may translate to different contacts for each of that plurality of subscribers. A table is maintained at the PTT SSS that correlates the alias numbers with the actual phone numbers, for each subscribing mobile station contact list. The alias numbers in the contact lists are subject to change, thus requiring the contact lists to be centrally administered in order to maintain correlation between the subscriber's named contacts and their phone numbers. Updated contact lists are downloaded to the mobile station resident clients when the updates occur. Because of the use of the contact list and aliases, the user typically does not know the number that the PTT client program uses to initiate a PTT communication and may not even remember the actual telephone number of the other party or parties.
For postpay subscribers there is no issue of concern, however, the use of the contact list and aliases raises concerns for prepaid operations, particularly when a prepaid mobile station is roaming off-net. A brief review of prepaid service may help to establish context for further discussion of PTT.
Many providers offering communication services through wireless mobile communication networks offer many of the services on a prepaid basis. As prepaid subscriptions to wireless communications services have become increasingly popular, prepaid availability has been extended to PTT service. Prepaid subscriptions are beneficial both to subscribers and the providers. The customer can budget an amount of airtime that will be used during a certain period of time to insure that the limited cost will not be exceeded. The wireless service provider receives payment in advance and avoids the risk that the customer will default on a bill that can occur with a postpay arrangement.
Conventionally, the prepaid wireless service customer purchases blocks of time for making voice telephone calls via a cellular telephone network. Upon connecting to the wireless communications network, the customer account is authorized and authenticated, and the network allows a call to proceed. The network monitors the customer's usage time and decrements from the customer's account. If the account becomes depleted, the system can either prompt the customer to purchase more time, or the system can terminate the call.
Prepaid network services offered by wireless carriers rely on intelligent network triggers to provide access to the carrier's prepaid server when a call is initiated to a called destination. The carrier's prepaid server times and rates a call originating from a location in the carrier's network, and the prepaid server deducts appropriate charges from the subscriber's prepaid account. The prepaid server can block or disconnect such a call if the subscriber is out of funds.
However, the use of intelligent network triggers or carrier specific techniques are not applicable when the prepaid subscriber is roaming “off-net.” For this reason, prepaid services rely on “hotlining” of calls originated by a prepaid subscriber roaming off-net to the home carrier's prepaid server. The serving off-net mobile switching center (MSC) that provides service to the roaming subscriber replaces the telephone number dialed by the subscriber with a number for the prepaid server, which was received by the serving MSC from the subscriber's home location register (HLR) when the roaming subscriber's mobile station registered with the serving MSC. Consequently, before a prepaid call can be completed successfully to the intended destination, the prepaid server must prompt the roaming subscriber to reenter the called number in order to extend the call further. With the destination number, the prepaid server can pass the call to a network switch for routing to the desired destination. The home carrier's prepaid server can then time, rate and control the call, which is forwarded for completion to the called destination. However, such processing of prepaid calls for roaming subscribers raises issues with regard to routing of voice calls for PTT type service based on contact lists and aliases.
With the “hotlining” of PTT calls originated to the prepaid platform when the subscriber is roaming “off-net,” the originally transmitted contact number information (alias) is replaced with the prepaid server number when the call is routed from the visited area MSC to the prepaid server. When the prepaid server prompts the roaming subscriber to reenter the destination number in order to extend the call further, the PTT customer is not likely to be able to readily respond to the reentry prompt in the context of a PTT call. Even if the selected contact is a single individual and the subscriber knows that contact's phone number, user input of that phone number would not properly direct the PTT call the PTT SSS. The PTT call thus would fail if the subscriber cannot reenter the correct PTT alias number.
As a result, carriers introducing voice call-based PTT services face a choice of either only offering PTT service to postpay type subscribers, or offering their prepaid subscribers a limited version of the PTT service that only allows prepaid subscribers to originate PTT calls when they are on-net. Neither of these options is commercially attractive.
Hence a need exists for a prepaid PTT service that can be implemented effectively for mobile calls that originate at roaming as well as on-net locations. Such a service should eliminate incompatibility between the roaming design of the voice call-based PTT services and the roaming design of the prepaid services being offered by many wireless carriers. System enhancements should avoid imposition of specially designed PTT handsets for prepaid subscribers, thereby enabling a wireless carrier to maintain only a single inventory of PTT handsets.